Memory devices such as Dynamic Random Access Memory (DRAM) and Double Data Rate Dynamic Random Access Memory (DDR DRAM) are widely used in computers and other electronic devices for the storage and retrieval of data. DDR DRAM devices use source synchronous transfers when data is written to or read from the memory devices. Second generation Double Data Rate (DDR-II) devices are defined by a JEDEC Solid State Technology Association Standard JESD90.
DDR-II devices require some form of externally supplied voltage to carry out various memory operations, including memory read, program and erase operations. Voltages of various magnitudes may be required to carry out these operations. Sometimes a situation may occur where the supply voltage of the particular system incorporating the DDR-II devices enters a low voltage condition. Typically, the supply voltage levels are permitted to vary by approximately 5-10% from the specified level and the DDR-II device will still operate correctly. However, if the supply voltage level falls outside the specified ranges, the data in the DDR-II device may become corrupted or lost.
If the supply voltage level falls below a specified level, the JEDEC Standard requires that DDR-II devices be properly reset. Accordingly, what is needed are apparatus and methods utilizing a brown-out protection circuit for effectively responding to a low supply voltage condition to prevent improper DDR-II device operation.